[Rimrock]

Just a couple of comments here; the rpm of the excavator swing of the 1:1 version and any scale model of it, will be equal. Joe's machine turns about 6-7 rpm, which is normal for a 50 ton + digger. Smaller machines like 320 do turn a bit quicker, say 10 rpm. Concerning gear sizes, one usually would start out determining the size of the final gear, the biggest, placed in the slewing ring, and then work one's way back and find a suitable gearmotor. The layout and design of your slewing ring will decide how big gear one can place there. 'Larger is better' one can say about the final gear placed in the slewing ring, as a bigger gear here reduce the strain of the tiny gears in the gearmotor.
If one got a 15 rpm gearmotor, then the final ratio will be 15rpm/10rpm slewing ring speed = 1.5 If the largest gear is 44 teeth, then the driven gear would be about 30 teeth (44/30 = 1.47 , 15 rpm / 1.47 = 10.2 rpm)